Perthiocyanic Acid, H2C2N2S3

Perthiocyanic Acid, H2C2N2S3, may be prepared by the action of mineral acids on thiocyanic acid. When potassium thiocyanate (100 parts) dissolved in water (60 parts), and slightly more sulphuric acid (100 parts, sp. gr. 1-44) than is required to liberate the thiocyanic acid, are mixed, on standing orange-coloured crystals of perthiocyanic acid separate out:

3HCNS = HCN + H2C2N2S3.

When heated above 220° C., perthiocyanic acid decomposes into carbon disulphide, ammonia, sulphur, and a residue of composition which varies according to the temperature. On hydrolysis with water the main reaction is: